The subject of the present invention is an electric window regulator for a motor vehicle, comprising a mechanism for driving the window, an electric motor that provides a motive force to the driving mechanism, provided with an anti-pinching system which can be triggered in the event of a hard obstacle interfering with the travel of the window, and an electronic circuit for control of the motor and of the anti-pinching system.
It is known that electric window regulators for motor vehicles are increasingly subject to regulations limiting the pinching force allowable between the window and the door frame to a maximum value. Such regulations address situations involving obstacles characterized by stiffnesses of 10, 20 or 60 N/mm, that is to say obstacles of varying degrees of hardness.
In a window regulator of the type with cables associated with one or two guide rails fitted with one or two runners supporting the window, springs are interposed between the motor unit and the ends of the sheaths enveloping the cables. The spring associated with the upper cable maintains the tension in it during lowering, and, during raising, is continuously fully compressed, under a transfer force of about 80 Newtons, while the lower spring tensions the corresponding cable during raising. This is because the stiffness of the helical springs usually used is from 2 to 5 N/mm, and these springs are produced in such a way as to reach end of travel when they are subjected to sufficient compression, under a force of 25 to 50 Newtons at most.
It can thus be understood that, in the event of a hard obstacle with stiffness greater than 20 Newtons per millimeter, these springs do not in fact fulfil the function of an impact damper. The impact and the pinching which ensue are of such abruptness that the electronic circuit cannot trigger the anti-pinching system as rapidly as might be wished. This may entail physical injury if a person""s hand or head is thus pinched between the window and the door frame of the vehicle.
A system comprising a damper made of flexible material is known from the patent U.S. Pat. No. 5,296,658, this damper being applied to the door seal and containing electrical conductors which come into mutual contact in the event of an obstacle interfering with the travel of the window, causing the damper to be squeezed. The electrical conductors are associated with a switch, opening of which causes the drive motor to be cut off. Such a device thus employs an electrical circuit but does not make any provision for effectively reducing the impact of the obstacle.
The patent FR-A-2 693 535 also describes an electromechanical system which detects the obstacle and cuts off a switch which causes the drive motor to be reversed. As in the device of the preceding patent, this system does not allow the stiffness to be reduced and employs electrical means.
The patent EP-A-0 579 518 describes a safety device for electric window regulators employing general means similar to those described in the abovementioned patents, but with a guide rail which can be shifted downwards in the event of an obstacle whose stiffness exceeds a predetermined value. This downwards shifting of the rail actuates electromechanical means which automatically reverse the direction of rotation of the motor.
The patent EP-A-0 604 272 teaches a window regulator device for a vehicle the kinematic linkage of which comprises a flexible element for force-limiting and damping a hard impact between the window and a hard obstacle. This flexible element is configured to limit the force to about 100 Newtons so as to form an anti-pinching safety system preventing the hard impact.
This flexible element may, for example, be a flat spring capable of bending upon a hard impact suffered by the window, to which corresponds a shifting of the elements constituting the window regulator.
These devices, however, exhibit the drawback of reaching end of travel at the maximum forces allowed for the pinching, that is to say 100 Newtons and even below this maximum force. It results therefrom that such springs to not fulfil a damping function, with the drawbacks already set out (impacts and abrupt pinching preventing the electronic anti-pinching circuit from triggering within the desired timescale, with the risk of physical injury).
Hence the object of the invention is to propose a satisfactory solution to this problem.
In accordance with the invention, the electric window regulator comprises mechanical means limiting the obstacle stiffness in order to reduce the pinching forces on hard obstacles, by mechanically converting the impact with a hard obstacle into the impact with a less hard obstacle while the window is rising, and the mechanical means are formed in such a way as not to reach an end of travel under a predetermined allowable maximum pinching force.
The invention applies to any type of window regulator equipped with an anti-pinching system on the motor: speed sensor, torque sensor, current-consumption reading, etc. In this type of device, the greater the stiffness of the obstacle, the higher the pinching force.
In the case of a window regulator of the type with a cable driving a window-support runner sliding on at least one guide rail, the mechanical obstacle-stiffness limiting means consist of a spring of the upper cable having a stiffness of at least about 10 N/mm and formed in such a way as to reach end of travel only under a compression force of least about 250 Newtons.
It is known that the stiffness K of a spring is defined by the following relationship:   K  =            F      +      100        d  
F being the force for raising the window, d the damping travel of the spring until it reaches end of travel under a compression force, and 100 being the number of-Newtons corresponding to the maximum pinching force allowable according to the European and American standards.
Hence, in the case of a window regulator of the type with a cable and guide rail, the spring associated with the upper cable in accordance with the invention is no longer at end of travel during the raising phase under a transfer force of about 80 N to 150 Newtons. Hence it then functions as a damper, converting a hard impact into a soft impact, in addition to its function of maintaining the tension in the cable. That being so, 4a defined torque value, higher than the normal operating torque of the motor while the window is being raised, is reached as a consequence of a hard obstacle only after a time interval longer than the time interval corresponding to a spring according to the state of the prior art, which would already be constantly fully compressed during the normal raising of the window with no obstacle. It results therefrom that the electronic circuit has a longer time in order effectively to trigger the anti-pinching system and thus to avoid any risk of physical injury.